Petalidium saxatile: A new species of Petal-bush from Namibia.

Petal-bushes, Petalidium spp., are woody perinal shrubs in the Family Acanthaceae found in arid sandy or stoney areas of Africa, India, and the Mascarene Islands, although most species are found in areas of Southern Africa with summer rainfall and no frosts. There are currently 35 species recognised from Southern Africa, of which 33 are found in Namibia, 13 in Angola, and six in South Africa. Despite the name 'bush', they are diverse in form, ranging from scrambling herbs to large, robust shrubs. Many species of Petalidium are fast-growing and produce attractive  flowers, leading to some species being cultivated by gardeners.

In a paper published in the journal Phytotaxa on 3 September 2025, Wessel Swanepoel of the H.G.W.J. Schweickerdt Herbarium at the University of Pretoria, Kyle Dexter of the School of GeoSciences at the University of Edinburgh, the Royal Botanic Garden Edinburgh, and the Department of Life Sciences and Systems Biology at the University of Turin, Martino Adamo, also of the Department of Life Sciences and Systems Biology at the University of Turin, Erin Manzitto-Tripp of the Department of Ecology & Evolutionary Biology, & Museum of Natural History at the University of Colorado, and Abraham Van Wyk, also of the H.G.W.J. Schweickerdt Herbarium at the University of Pretoria, and of the South African National Biodiversity Institute, describe a new species of Petalidium from the Kaokoveld Desert of northwestern Namibia.

The new species is described on the basis of four populations of Petalidium in the Kaokoveld discovered by Wessel Swanepoel while carrying out research for a monograph on the genus in Southern Africa. The populations had been observed by him before, and assumed to belong to the species Petalidium canescens, which is also found in the region. However, in May 2018 Swanepoel visited a population near Palmwag while it was in flower, suggesting that it was a different species, something which was later backed up by generic analysis.

The new species is named Petalidium saxatile, where 'saxatile' derives from the Latin 'saxatilis', meaning 'found amongst rocks', from the habitat where it lives. These Plants form a hemispherical woody shrub about 50 cm tall, with bifurcating stems and elongate oval leaves, both covered with white hairs. Flowers are roughly tubular, with four lobes, the lower of which is yellow and the others pinkish brown (unlike the all pink flowers of Petalidium canescens).

Petalidium saxatile, morphology of flowers. (A), (B) Newly opened and faded flowers. Anterior corolla lobe inside (adaxially)  bright yellow and without nectar guides. (C) Flower viewed from above, showing puberulous abaxial surface of posterior corolla lobes. (D) Flowers viewed obliquely from above, all corolla lobes discolorous with the outside (abaxial) surfaces puberulous and notably paler in  colour. The long white trichomes next to the flower on the right do not belong to the plant, but is a wind-blown feathery awn (arrowed) of  a member of the Grass genus Stipagrostis. Wessel Swanepoel in Swanepoel et al. (2025).

Petalidium saxatile is known from only four sites, three in an approximate line from Palmwag southwards to Bergsig, and one on Welbedacht Farm, to the south of Khorixas. It has been found growing among rocks from the Etendeka Group basalt, on arid hillsides and along drainage lines at elevations of 860–1130 m above sealevel, between 70 and 150 km from the sea. Searches of other areas with similar conditions did not reveal any further populations. It is calculated that the area of potential habitation for Petalidium saxatile is 3337 km², but the area of actual habitation is less than 20 km². All of the areas where the species is found are prone to prolonged droughts. As such, Swanepoel et al. recommend that Petalidium saxatile be classified as Endangered under the terms of the International Union for the Conservation of Nature's Red List of Threatened Species. 

Known distribution of Petalidium saxatile (black dots). Swanepoel et al. (2025).

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Petalidium hoarusibense: A new species of Petal-bush from Namibia.

Petal-bushes, Petalidium spp., are woody perinal shrubs in the Family Acanthaceae found in arid sandy or stoney areas of Africa, India, and the Mascarene Islands, although most species are found in areas of Southern Africa with summer rainfall and no frosts. There are currently 32 species recognised from Namibia, 13 from Angola, and six from South Africa. Despite the name 'bush', they are diverse in form, ranging from scrambling herbs to large, robust shrubs. Many species of Petalidium are fast-growing and produce attractive  flowers, leading to some species being cultivated by gardeners.

In a paper published in the journal Phytotaxa on 8 January 2025, Wessel Swanepoel of the H.G.W.J. Schweickerdt Herbarium at the University of Pretoria, and Abraham Van Wyk, also of the H.G.W.J. Schweickerdt Herbarium at the University of Pretoria, and of the South African National Biodiversity Institute, describe a new species of Petalidium from the Kaokoveld region of northwestern Namibia.

During a visit to the Okandjombo area of the Kaokoveld in November 2023, Wessel Swanepoel obseeved a number of distinctive Petalidium Plants, with a dwarf-shrubby habit, pealing white bark, and multiple stems arrising from just below the ground level. At this  time the Plant was not in flower, making it difficult  to confirm that it was not a previously described species, however, during a follow-up visit in May 2024, the plants were found to be producing distinctive pink flowers, confirming it as a  new species. This is named Petalidium hoarusibense, in reference to the Hoarusib River, as it was discovered within the catchment of this river.

Petalidium hoarusibense. Habitat and habit. (A) Mature plant about 800 mm high (greyish shrub in foreground), growing among dark grey sedimentary rocks of the Swakop Group along the banks of the Hoarusib River near Okandjombo, Namibia. (B) Base of plant showing several branches arising from the main stem just above ground level, each covered with rough, fissured, greyish white bark. Wessel Swanepoel in Swanepoel & Vak Wyk (2025).

To date, Petalidium hoarusibense, has only been found in m the mountainous area along the Hoarusib River and its tributaries to the east and south of Okandjombo, part of the Great Escarpment of northwestern Namibia, on on arid hillsides and along drainage lines at elevations of between 450 and 700 m above sealevel. This area recieves about 100 mm of rainfall in an average year, and additionally has about five foggy days, which might contribute to the available moisture slightly..

Petalidium hoarusibense. Morphology of leaves and flowers. (A) Shoot with flower and leaves, the latter with dense, greyish white indumentum. (B), (C) Flower in front (B) and side (C) view; note bracteoles (some of spent flowers) with cobwebby indumentum. (D), (E), (F), (G) Flowers in front view, each from a different plant to show variation. Note all corolla lobes of a flower being roughly similarly coloured in various shades of magenta, or lateral and upper lobes slightly darker shaded than the anterior lobe, sometimes, as in (D), (E), & (G) with darker magenta or maroon triangular nectar guides; anterior lobe adaxially with two narrowly triangular yellow nectar guides. Wessel Swanepoel in Swanepoel & Vak Wyk (2025).

All known specimens of Petalidium hoarusibense were found growing within an area of about 20 x 18 km, although within this area there were several areas where it was quite common. There were also a large number of dead Plants on the hillsides, possibly due to a prolonged drought in the area. For this reason, Swanepoel and Van Wyk reccomend that the species be listed as Vulnerable under the terms of the International Union for the Conservation of Nature's Red List of Threatened Species.

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Sesamothamnus leistneri: A new species of Sesame Tree fromthe Kaokoveld Region of Namibia and Angola.

The Sesame Family, Pedaliaceae, contains eleven genera of shrubs and trees, including the True Sesames, Sesamum spp.. Eight of these genera are confined to Africa, where the family is thought to have originated, while one, Uncarina spp., if found only on Madagascar. Of the remaining two genera, Pedalium (which contains only a single species, Pedalium murex) is found across much of Africa and South and Southeast Asia as far east as Java, while Sesamum spp. has a near global distribution, although it is likely to have been introduced to many places by Humans, due to the use of its seeds as a food crop.

Phylogenetic studies of the Family Pedaliaceae have suggested that the basalmost genera is Sesamothamnus, the Sesame Trees or Sesame Bushes, a group found in arid areas of Africa, split into two groups with a bimodal distribution. The first group of species is found in East Africa, from Ethiopia and Somalia south into Kenya and Tanzania, and are large, tree forming plants, the second group has a Southern African distribution, being found in Angola, Namibia, Botswana, Zambia and Zimbabwe, and are smaller, shrubby plants, with a bulbus base from which a single, taperinc stem arises.

The most recent species of Sesamothamnus to have been described was Sesamothamnus lugardii in 1906, however, in 1957 the South African botanists Bernard de Winter and Otto Albrecht Leistner recorded seeing a tree-like species of Sesamothamnus while on an expedition to the Kaokoveld Region of northwestern Namibia, and in addition brought back sterile samples which were stored in the collections of the South African National Biodiversity Institute in Pretoria and the National Botanical Research Institute in Namibia, where they were tentatively identified as Sesamothamnus benguellensis, a shrub-like form found in Namibia and Angola. In 1966 the Namibian botanist Heinrich Johann Wilhelm (Willy) Giess collected fertile specimens of the tree, which were stored in the National Botanical Research Institute in Namibia under the name 'Sesamothamnus leistneranus' apparently in honour of Otto Albrecht Leistner, although he never published a description of this material.

In a paper published in the journal Phytotaxa on 14 November 2023, Wessel Swanepoel, an independent researcher from Windhoek and Abraham van Wyk, from the H.G.W.J. Schweickerdt Herbarium at the University of Pretoria, formally describe the Kaokoveld Sesame Tree as a new species.

The species is described as Sesamothamnus leistneri, rather than Sesamothamnus leistneranus, as this implies that it was discovered by Leistner, rather than simply named in his honour. Wessel Swanepoel and first observed an unfamiliar Sesame Tree while visiting the Kaokoveld in 1990, although at that time the tree was not in flower or fruit, making identification very hard. In 2005 Swanepoel and van Wyk observed another Sesame Tree in the Otjitanda area of the Kaokoveld. This time the tree was producing both flowers and fruit, and samples were taken which enabled it to be compared to the material previously collected by de Winter, Leistner, and Giess.

Sesamothamnus leistneri, habitat and habit. (A) Multi-stemmed tree with distinct browse line caused by domestic stock. (B) & (C) Trees showing typical upright habit and relatively dense green foliage during the rainy season. Swanepoel and van Wyk (2023).

Sesamothamnus leistneri is a single or (more-commonly) multi-trunked succulent tree, reaching about 8 m in height. Branches are formed high on these trunks, and covered with sharp spines up to 30 mm in length, formed from modified petioles (leaf stalks). The bark of the tree is creamy yellow, and peels away from the trunks in papery strips. The trees are deciduous, producing waxy green leaves 40-70 mm in length and 15-40 mm. Flowers are produced from January to June and are large and cylindrical, reaching 150 mm in length and 55 mm in width, white or cream on the outside and bright yellow on the interior. Flowers of this type are likely to be pollenated by Long-tongued Hawkmoths, as is the case with other members of the genus. The fruit, produced from February onwards is a green-to-yellow rigid woody capsule, 65-90 mm in length and 25-32 mm in width, which splits open during the dry season releasing a large number of small, winged seeds.

Sesamothamnus leistneri, habitat and habit. (A) Typical single-stemmed tree. (B) Tree with foliage starting to turn yellow in autumn. (C) Leafless tree during the dry season (winter), with persistent dehisced fruit. Swanepoel & van Wyk (2023).

Sesamothamnus leistneri was found growing in mountainous regions on both sides of the Kunene River, which forms the border between Namibia and Angola. In Namibia it was found from the Ehomba Mountain and Zebra Mountains in the east throughout the Baynes Mountains to the Otjihipa Mountains in the west, and south as far as Sesfontein. In Angola Sesamothamnus leistneri was observed near the summit of Serra Tchamalindi in the Iona National Park, and is thought likely to be fairly widespread.

Sesamothamnus leistneri, vegetative morphology. (A) Single-stemmed tree showing fluted trunk. (B) Bark peeling in papery pieces. (C) Long shoot with leaves clustered on lateral short shoots. (D) Mature leaves showing green colour and glossy upper surface. (E) Long shoot with spines derived from petioles; one dried leaf blade still attached. Swanepoel & van Wyk (2023).

Sesamothamnus leistneri grows on rocky mountain and valley slopes, in kloofs (gorges), less often on plateaus and at the base of hills, in Colophospermum-Commiphora woodland at elevations of 1000–1600 m, Between 83 and 220 km from the Atlantic Ocean. Rainfall in this area is typically 100-300 mm per year. This area is sparsely populated by Humans, and the tree does not appear to be utilised by Humans in any way, though it is browsed by livestock during periods of drought. As such Swanepoel and van Wyk assess that it should be classified as being of Least Concern under the terms of the International Union for the Conservation of Nature's Red List of Threatened Species.

Sesamothamnus leistneri, flower morphology. (A) Inflorescence showing one open flower, several flowers in different stages of development, and persistent styles (whitish, tipping blackish ovaries) of faded flowers following shedding of the corolla. (B) Flowers, open and in bud; note both stamens and stigma positioned in mouth of the corolla tube, also the lack of a spur at the base of the tube. (C) Flower showing style with bilobed stigma exserted about 10 mm from the mouth of the corolla tube. (D) Distal part of corolla tube opened out to show the yellow inner surface of the tube, white filaments, and anthers positioned at the level of the corolla mouth. Swanepoel & van Wyk (2023). 

The tree-like general habit of Sesamothamnus leistneri suggests that it is more closely related to the Sesame Trees of northern East Africa than the shrubby forms of Southern Africa. This is not altogether surprising, as while these areas are now separated by humid tropical forests where these Plants are unlikely to be able to survive, it is thought that the two zones would have been connected by bands of arid land several times during the Pleistocene glacial phases, when the climate of Africa would have been cooler and dryer.

Sesamothamnus leistneri, fruit morphology. (A) Two almost mature fruit and a dehisced one. (B) Old dehisced fruit persist for some time on trees. Swanepoel & van Wyk (2023).

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